This invention relates to an amplifier circuit for controlling the magnitude of the output power of a radio frequency (RF) signal.
In order to transmit RF signals an RF transmitter amplifier is generally coupled to an antenna which radiates the signal so that it can be intercepted by appropriate receivers. The RF power developed by the transmitter amplifier and coupled to the antenna is commonly referred to as "forward power". However, since the transmitter amplifier and antenna are not electrically ideal elements, a proportion of the forward power will in practice be reflected back to the amplifier.
By incorporating a circuit which controls the magnitude of the output power the transmitter amplifier can be protected against excessive forward or reflected power which might otherwise destroy the amplifier. For example U.S. Pat. No. 3,449,680 provides protection by sensing the forward power to the final amplifier and reducing the current to a preceding stage if the final amplifier current increases. This takes no account of the reflected power. On the other hand, detection circuits have been used to sense increases in reflected power, for example when the transmitter amplifier sees an open circuit, e.g. because the antenna has been disconnected, broken off, or incorrectly assembled and to reduce the power of the transmitter amplifier accordingly. However, such circuits provide no protection against increases in forward power.
U.S. Pat. No. 3,641,451 discloses a circuit which protects against both forward and reflected power excesses. In that case a detector senses the forward power level and generates a first signal indicative thereof. This first signal is compared with a reference signal to provide a control signal which determines the power developed by the transmitter amplifier. Any increase in the forward power level is detected which changes the control signal to cause a reduction in the forward power developed by the transmitter amplifier. Conversely, any decrease in forward power level is detected and results in changing the control voltage to cause an increase in forward power developed by the transmitter amplifier. The reflected power is also sensed in such manner as to generate a second signal. When the reflected power is less than a predetermined threshold value no compensation is made to the power output. However, when the reflected power exceeds the threshold value this second signal causes a reduction in the magnitude of the reference signal, thus altering the control signal which consequently reduces the forward power of the transmitter amplifier to a safe level. Hence, above the threshold value, the forward power developed by the transmitter amplifier is decreased as the reflected power increases and vice versa, whereas below the threshold value the forward power is not modified to take account of the reflected power.
U.S. Pat. No. 3,641,451 employs a single threshold value for the reflected power at which the power reduction mechanism cuts in. However, the reflected power may in practice increase safely when the forward power is increased. No allowance is made for this in that U.S. patent, and therefore the gain of the transmitter amplifier may be reduced unnecessarily when the forward power is increased. With a view to overcoming this particular problem U.S. Pat. No. 4,122,400 proposes means for determining the ratio of the reflected power to the forward power and only when this ratio exceeds a given threshold is the gain of the amplifier reduced to protect it. The value of the reflected power at which the protection mechanism cuts in is variable depending on the magnitude of the forward power. This clearly permits a proportionally greater reflected power before the protection mechanism cuts in. Once again, above the threshold value, the forward power developed by the transmitter amplifier is decreased as the reflected power increases and vice versa, whereas below the threshold value the forward power is not modified to take account of the reflected power.
British patent application GB-A-2,019,142 and U.S. Pat. No. 4,547,746 both disclose transmitter circuits in which means are provided between the final filter and the antenna (i.e. after the filter) for sensing both the forward power and the reverse power. The two signals thus obtained are subtracted to provide a control signal for adjusting the output of the power amplifier.
It is noted that in the prior art, the power output of the final amplifier is generally reduced as the reflected power increases and vice versa.
The prior art circuits mentioned above tend to maintain the output power of the transmitter amplifier at a substantially uniform predetermined magnitude. Circuits which achieve this effect are commonly known as automatic levelling control or automatic output control circuits. Conventionally, transmitter amplifiers have only been required to operate at one power level. Such transmitter amplifiers are adjusted during manufacture to maintain the desired output power level. In some applications, it was desirable to have two power settings, specifically a high and a low level. The transmitter amplifier for such applications would have one setting for the high power level and another for the low power level.
Recently, with the advent of cellular radio telephones, it has become advantageous to re-use radio channels. Therefore, in order to avoid interference between two RF transmitter amplifiers using the same frequency, it is desirable to have a plurality of different selectable output power levels. Automatic output control circuits enabling an RF transmitter amplifier to maintain one of a plurality of output levels selected in response to control signals are disclosed in U.S. Pat. Nos. 4,523,155 and 4,602,218.
The transmitter amplifier of a radio is generally coupled to the antenna via a filter. It is an inherent feature of the filter that the transmitted power is, to a limited extent at least, frequency dependant. A plot of transmitted power versus frequency will show an oscillation or so-called "ripple" in the power level across the frequency band, as shown by plot A in FIG. 2 of the accompanying drawing. Thus, although in the prior art the power is levelled on the amplifier side of the filter, the power on the antenna side of the filter will vary depending on the frequency.
Ideally, the power on the antenna side of the filter should be substantially uniform irrespective of frequency, but the prior art documents discussed herein do not address this problem.